USACO1.3:milk

#include<stdio.h>int m,n,i,j;int p1[5000],p2[5000];int sum,ans=0;int main(){    int max=0,k,r;    freopen("milk.in","r",stdin);    freopen("milk.out","w",stdout);    scanf("%d%d",&n,&m);    for(i=0;i<m;i++)                  scanf("%d%d",&p1[i],&p2[i]);    for(i=0;i<m-1;i++)        for(j=i+1;j<m;j++)            if(p1[i]>p1[j]){                k=p1[i];p1[i]=p1[j];p1[j]=k;                r=p2[i];p2[i]=p2[j];p2[j]=r;                      }    max=0;    for(i=0;;i++){      if(max+p2[i]<n){            ans+=p1[i]*p2[i];            max+=p2[i];          }else{            ans+=p1[i]*(n-max);break;        }    }printf("%d\n",ans); }